Halogen states and colours down Group VII
Learn the halogens at room temperature: fluorine pale-yellow gas, chlorine yellow-green gas, bromine brown (orange-brown) liquid, iodine grey-black solid. Down the group state runs gas to liquid to solid, so melting/boiling points rise and colours darken. Volatility (how readily a vapour forms) decreases down the group. Extended: heavier molecules have more electrons and stronger intermolecular forces, needing more energy to separate.
Reactivity falls; more reactive displaces
Halogens gain one electron to form a halide ion (Cl2 + 2e- -> 2Cl-); this gets harder down the group, so reactivity falls: Cl2 > Br2 > I2. A more reactive halogen displaces a less reactive halide from solution: chlorine added to KBr turns orange-brown as bromine forms, Cl2 + 2KBr -> 2KCl + Br2. Chlorine and bromine also displace iodine from KI; iodine displaces neither. Cyclohexane confirms: orange for bromine, violet for iodine.
(Extended) Why reactivity falls down Group VII
A halogen gains an electron into its outer shell. Down Group VII the atomic radius grows and more shells shield the outer electron, so despite rising nuclear charge the pull on an electron falls; the atom gains an electron less readily and is less reactive: fluorine (2.7) gains one easily (most reactive), iodine far more reluctantly. In displacement the halide ion is oxidised (loses electrons), the halogen reduced (gains them).
Drawn from real examiner reports.
Iodine is a grey-black solid
At room temperature iodine is a grey-black solid - a much-missed fact. Two wrong answers recur: purple/violet gas (that colour is the vapour formed on warming, not the solid) and brown liquid (that is bromine). Warmed gently, grey-black iodine sublimes straight to a purple vapour with no liquid stage. If the question says at room temperature, answer grey-black solid.
S23 P32 Q1(d)
Compare halogen with halogen, not the salt
Explaining displacement, candidates write chlorine is more reactive than potassium bromide - comparing an element with a salt, scoring nothing. The halogen reacts with the halide ion (Br-); potassium is a spectator. Say chlorine is more reactive than bromine, displacing bromide to give bromine and KCl. Compare the two halogens directly.
S22 P31 Q3(c)(ii); S23 P31 Q2(b)(ii)
The halide ion is oxidised
In a displacement like Cl2 + 2Br- -> 2Cl- + Br2, candidates name the wrong species. The halide ion is oxidised: Br- loses an electron (-1 to 0). The halogen is reduced: Cl2 gains electrons (0 to -1). Rule: halide oxidised, halogen reduced. Name the species and define oxidation as loss of electrons; 'changed' alone scores nothing.
S22 P41 Q7(d)(ii)
Group VII reactivity falls going down
Group VII is the opposite of Group I: reactivity decreases down the group, because the atom gains an electron less readily as the outer shell moves out and is more shielded. Students who learned that Group I reactivity increases carry that direction across and wrongly write increases for halogens. Chlorine is more reactive than bromine, then iodine.
Volatile is not the same as reactive
Volatility and reactivity differ. Volatility is how easily a vapour forms, set by intermolecular forces and boiling point. Reactivity is how readily an atom gains an electron, set by atomic radius and shielding. Both fall down Group VII, so students merge them - but a volatility question wants the boiling-point argument, not electron-gain.
Answer displacement in three parts
Give a displacement answer in three parts: (1) colour change - orange-brown for Cl2 + KBr, brown for iodine, or none if no reaction; (2) which halogen formed; (3) conclusion - the added halogen is more reactive. Colour alone scores one mark.
Ionic equations: cancel the spectators
Start from the full balanced equation and cancel the potassium ions unchanged on both sides: Cl2 + 2Br- -> 2Cl- + Br2. Check atoms and charge (2- each side), keeping the 2 on the halide. K+ is a spectator, never in the final ionic equation.
Use cyclohexane to tell Br2 from I2
Bromine and iodine both look brown in water, so to tell them apart add cyclohexane and shake: the halogen moves into the organic layer - orange for bromine, violet for iodine. Quote this layer's colour when the aqueous colours are ambiguous.
Halogen — a non-metal element in Group VII of the Periodic Table. The name comes from Greek, meaning "salt-former". Halogens exist as diatomic molecules (two atoms bonded together): , , .
Volatility — how readily a substance vaporises. A highly volatile substance has a low boiling point and readily evaporates at room temperature. Down Group VII, volatility decreases (chlorine is a gas; iodine is a solid at room temperature).
Displacement reaction — a reaction in which a more reactive element takes the place of a less reactive element in a compound. A more reactive halogen displaces a less reactive halide ion from solution.
State the physical state and colour of chlorine, bromine, and iodine at room temperature.
Chlorine gas is bubbled through a solution of potassium bromide.
(a) Describe what you would observe.
(b) Write a balanced symbol equation for the reaction. Include state symbols.
(c) State what this experiment shows about the relative reactivity of chlorine and bromine.
